This is an automatically generated Delivery Status Notification.
Delivery to the following recipients was aborted after 13.9 hour(s):
* recipients address
Reason: The following message to recipients address was undeliverable.The reason for the problem: Maximum Retry Queue Age Reached

If a mail sender finds that a recipient mailbox is temporarily unavailable, for example, the mailbox is full or there's a communication issue with the recipient's mail service that might be quickly fixed then the sender will queue the mail and try again later to send to the recipient.

There comes a point when the sender gives up trying and this is when the time allowed for retrying to deliver the mail has exceeded the time set for this process in the mail sender settings. The mail will be removed from the retry queue and the mail user gets the message.

There may be an additional error code to indicate why the mailbox or recipient's mail service is unavailable.

but these are due to emails NOT being received by the recipients - rather than emails NOT being sent to me !

...so let's start from the basics.

You compose an email and press send. That message goes from your outbox in the mail client otherwise known as the mail user agent to the mail sender / outgoing server or mail delivery agent for onward transmission via mail transfer agents to the recipient's mail service for delivery to the recipient's mailbox.

There are queues for email sending and email receiving whilst the systems communicate to get your email to the recipient's mailbox.

So when I say sender I'm not talking about you I'm talking about the hardware and software stuff on the sending side of the email system that's trying to deliver your email.